1. Field of the Invention
This invention relates to a process for producing a porous material having improved mechanical strength, uniform fine pore size and superior permeability from a solution of a vinylidene fluoride/tetrafluoroethylene copolymer or a vinylidene fluoride polymer. More specifically, it provides a process for producing a porous membrane having a fine pore size which is useful, for example, for membrane filtration, ultrafiltration and dialysis.
2. Description of the Prior Art
Generally, porous materials are produced by methods such as mixing a blowing agent, blowing gases, mechanical bubbling, solvent or extraction of a resin. The resulting porous materials, however, have a large pore size with non-uniform pores, and become closed celled, and their properties are not satisfactory for practical purposes.
Known methods for producing porous materials using a solvent include those for the production of artificial leathers using a polyurethane-type material and for the production of cellulose membranes, polyvinyl chloride, or polyamides.
In the production of artificial leathers, pore size is scarcely controlled, and such a material is not suitable for use as a membrane.
Porous materials with a controlled pore size are mostly made of cellulose derivatives, and are used for various filtration purposes and dialysis, for example. Membranes made of cellulose derivatives, however, suffer from the defect that because of their inherent properties they are easily attacked by acids, bases or organic solvents, and, moreover, deform under heat or pressure, resulting in deteriorated properties. This has considerably limited the conditions under which these cellulosic membranes can be utilized.